A planographic printing plate material is well known which comprises a support subjected to hydrophilic treatment and provided thereon, a photopolymerizable layer and a protective layer. Recently, a method of manufacturing a planographic printing plate is carried out which subjects a planographic printing plate material to digital exposure employing a laser based on image information, and developing it, whereby a printing plate with an image of high resolution is obtained at high speed without employing an original film. For example, a process of preparing a planographic printing plate is known which comprises scanning-exposing a planographic printing plate material to light modulated by output signals from an electronic plate making system or an image processing system or by image signals transmitted through channel.
It is generally known that the photopolymerizable layer as described above contains an acryl monomer, an alkali-soluble resin, a photopolymerization initiator, and optionally a sensitizing dye to absorb a range of wavelengths of emitted light for exposure (particularly laser light).
As a light source for exposing a photopolymerization type light sensitive planographic printing plate material is used a visible light source having a longer wavelength such as an Ar laser (488 nm) or an FD-YAG laser (532 nm). In recent years, semiconductor lasers employing, for example, InGaN type material, which can continuously emit light with a wavelength of from 380 to 430 nm, are about to be put into practical use. A scanning exposure system employing light with such a short wavelength as a light source has advantages in providing sufficient output power and an economical system, since the semiconductor lasers can be manufactured at low cost. Further, a light sensitive planographic printing plate material to be applied to the above light source has a spectral sensitivity shorter than that of a planographic printing plate material to be applied to a system employing a conventional FD-YAG laser or Ar laser, and therefore provides an excellent safe light property, whereby operation under room light is easy. For the reasons above, a manufacturing method of a planographic printing plate has been strongly desired in this industry, which employs a relatively short wavelength semiconductor laser with a wavelength of from 350 to 450 nm.
Ketocoumarin compounds are disclosed in Japanese Patent O.P.I. Publication Nos. 2003-21901 as sensitizing dyes, which are used in a photopolymerizable composition having high sensitivity to a short wavelength semiconductor laser with a wavelength of 350 to 450 nm. However, these ketocoumarin compounds have problems in that sensitivity is insufficient and sufficient printing durability is not obtained.